Source
In this post I will be outlining two different theories on how drug addiction develops. Before we begin I first need to define positive and negative reinforcement.

Positive Reinforcement: The addition of a stimulus or event that will increase the frequency of a certain behavior or response.
Negative Reinforcement: The removal of a stimulus or event that will increase the frequency of a certain behavior or response.

Why does drug use continue even after these results? source
In these following theories of drug addiction, the behavior that is increased will be the frequency of drug use.

The Paradox of Drug Addiction

psychpost | 8/06/2012 02:00:00 PM | 6 Comments
We know the risks associated with drug use, but why do we continue to take them? Source

There are many drugs which are capable of damaging our bodies, our relationships with others, and can cause legal and financial issues.We are aware of the possible consequences, yet we have a large number of people who are addicted to drugs.

This raises the question: How can a person develop and maintain a pattern of drug use that is clearly destructive to the individual's life?

This paradox is seen with many different behaviors other than drug use (ie. overeating). In my next post I will be presenting two theories on how we believe drug addiction develops. One theory will focus on alleviating withdrawal symptoms and the other will focus on the rewarding effects of dopamine in the mesolimbic pathway (nucleus accumbens).
We often hear about withdrawal symptoms in regards to various drugs but we may not understand how these develop. Normally our body and brain are able to regulate all conditions within our body (ie temperature, excitability, energy, movement, mood, pain, etc) and keep them in a normal set point, which I will refer to as the baseline. 
 
Some common withdrawal symptoms of sedatives, stimulants, and opiates. Open for full size.
 
Drugs are able to alter these conditions in our body by altering the activity of the cells that control  these conditions. For example, one way Opiates relieve pain is by reducing the firing rate of neurons in the spinal column that are responsible for sending pain signals into the brain.

When one of these conditions are altered by a drug, the neurons must compensate and will make attempt to reverse the change. These changes in the neurons persist even after the drug is no longer in the body and no longer producing effects. These changes result in an effect opposite to the altered condition.

Visual representation of the compensatory change in baseline activity with drug use. Open for full size.
This information above may be difficult to understand so I have an example: An individual uses a psychomotor stimulant such as cocaine and experiences the effects of increased motor activity and increased alertness. There will be a change in the neurons that will function to bring these conditions back to a normal rate (decreasing them). When the drug is no longer in the body, these changes will persist and the individual may experience a high level of tiredness, which is opposite to the effects of the drug.